This invention relates to an electric motor driven vehicle and more particularly to one having an improved regenerative braking system for a DC shunt powered driving motor therefore.
As shown in Japanese Published Patent Application Hei 10-309005 (A) it has been proposed to employ a DC shunt motor for the driving of electric powered vehicles such as golf carts and the like. There the armature coil and the field coil are connected in parallel to a common electric power source. As is known, it is possible to energize the armature coil and the field coil independently from each other. The amount of current supplied to the armature coil is controlled based on the position of a operator controlled vehicle speed control such as, for example, an accelerator pedal. Then a specific current is supplied to the field coil depending on the armature current value. This is generally done by reference to a field map that is constant or pre-designed for each motor. This produces a specific torque from the electric motor to control the operation required by the various operating conditions of the electric vehicle.
In these types of electric vehicles there are also provided regenerative braking functions according to certain driving conditions. For example it is common to decelerate the vehicle to a specified speed without freewheeling when the accelerator control is released. In addition an arbitrary speed may be limited on a downhill run with the accelerator control remaining on. Also it is common to detect motion of unattended vehicle and apply braking of the vehicle and/or switch traveling direction between forward and reverse with the accelerator control remaining on during traveling. These controls are conventionally performed usually by controlling the voltages applied to the armature and field coils.
However, the voltage and capacity of the battery change according to conditions of use. Thus the torque of regenerative braking changes with the voltage and capacity of the battery and stabilized regenerative operation cannot be obtained. That is, as the characteristic among revolution speed (N), torque (T), and current (I) of the motor changes along with the battery voltage, it would be necessary to make a map of N-T-I characteristic for each battery voltage. This makes a control program complicated, requiring a large memory capacity and a large-sized circuit.
As an alternative in U.S. Pat. No. 6,686,712 it has been proposed to use various regenerative controls are performed according to vehicle speeds and independently of the position of the accelerator control. This, however, can give rise to a situation that control performed could be different from the intention of the operator. For example, if the vehicle decelerates on an uphill run, the armature current could become excessive even if the position of the accelerator control does not call for it.
It is, therefore, one principal object of the invention to compensate for the regenerative braking operations depending on the status of the battery depending on its voltage and capacity.
It is another principal object of the invention to provide vehicle speed control in response to the operator's intentions.
It is a still further object of the invention to provide a shunt motor control capable of maintaining constantly stabilized motor torque regardless of changes in voltage and capacity of the battery.